All tests today is with openSUSE TW with 4.2.1-1-desktop kernel. I changed only version of sg3_utils. In openSUSE TW with sg3_utils 1.41 'sg_vpd -p 0x0 /dev/sdX' takes no time, if is executed after displaying USB flash in desktop; but may take minute if executed before showing in desktop. `/usr/bin/sg_inq --export --page=sn /dev/sdc` takes _always_ a minute with sg3_utils 1.41: both before and after showing device in dekstop.